The main system RAM is allocated as needed, so technically almost all of the 2GB could be used. However, there are other qualifications on that RAM, such as being physically contiguous…so perhaps it should instead have the specification that “the largest physically contiguous block of RAM” can be used. I don’t remember what the kernel command line is for this, but I believe a contiguous block can be reserved at boot time via a kernel parameter if needed…someone else will have to say what kernel parameter reserves and binds that block to GPU.